Two hypotheses relating to the postnatal development of ammoniagenesis and acid production in rats will be tested. The first of these is that glutamine fails to provide ammonia because permeability barriers in the neonate prevent adequate penetration into cells and mitochondria. The penetration of glutamine into cells of kidney slices as well as in vivo will be investigated. The in vivo studies will involve micropuncture experiments and measurements of A-V differences. In addition, possible dependence of slice permeation on (OH/H) ratio will be determined. The effect of acidosis on penetration will also be (H) studied in animals at different stages of development. Comparable investigations will be carried out on mitochondrial permeability. A second series of experiments is related to the hypothesis that HCO-3 activated ATPase is important in acid production and in postnatal development of the acidification process. Brush borders will be isolated by sucrose gradient centrifugation from animals of different ages and the activity of the enzyme will be investigated. Possible relations between acid-base status and enzyme activity will be investigated.